Embodiments of the inventive concept relate to semiconductor memory systems, and more particularly, to operating methods for memory systems including both non-volatile Random Access Memory (RAM) and NAND flash memory.
Data is stored in NAND flash memory according to a number of threshold voltage distributions, each respectively associated with a given data state. That is, the memory cell transistor of a NAND flash memory cell may be “programmed” to a particular voltage level that falls within a prescribed threshold voltage distribution in view of a desired data state. There are many different programming approaches that may be used to adjust (up and/or down) a NAND flash memory cell threshold voltage. There are also many different memory cell and memory system related effects and phenomena that can undesirably change a NAND flash memory cell threshold voltage. When the programmed threshold voltage of a NAND flash memory cell changes beyond the bounds of a prescribed threshold voltage distribution, a read error (or “read fail”) may arise during a subsequent read operation.
In response to a read fail, many contemporary memory systems will execute a so-called “read retry operation”. During a read retry operation, a NAND flash memory cell may be iteratively read using different (e.g., variably-adjusted) read voltages. Read retry operations often resolve read fails, but require a significant amount of time. Thus, while read retry operations facilitate data accuracy they tend to reduce memory system operating efficiency.